1. Field of the Invention
The present invention relates to a piezoelectric filter, an antenna duplexer, and a communications apparatus employing a piezoelectric resonator for use in a wireless circuit for mobile communications, such as a mobile telephone, a wireless LAN or the like.
2. Description of the Background Art
Smaller size and lighter weight are required for parts incorporated in electronic apparatuses, such as mobile apparatuses and the like. For example, a filter or an antenna duplexer for use in a mobile apparatus needs to have small size, fine adjustment of frequency characteristics, and low insertion loss. An example of a filter satisfying such requirements is a piezoelectric filter employing a piezoelectric resonator.
FIG. 10 shows an exemplary structure of a conventional piezoelectric filter 500 employing a piezoelectric resonator. A portion (a) of FIG. 10 shows the piezoelectric filter 500 as viewed from the top. A portion (b) of FIG. 10 shows a cross-sectional view of the piezoelectric filter 500, taken along line E-E. A portion (c) of FIG. 10 shows the piezoelectric filter 500 as viewed from the bottom.
In FIG. 10, a first piezoelectric resonator 501 and a second piezoelectric resonator 502 are formed on the same substrate 503. The first piezoelectric resonator 501 is formed by successively providing a lower electrode layer 506a, a piezoelectric material layer 507a, an upper electrode layer 508a, and a frequency adjustment layer 509a via an insulating layer 505 on a first cavity 504a provided in the substrate 503. Similarly, the second piezoelectric resonator 502 is formed by successively providing a lower electrode layer 506b, a piezoelectric material layer 507b, an upper electrode layer 508b, and a frequency adjustment layer 509b via the insulating layer 505 on a second cavity 504b which is provided in the substrate 503. The second cavity 504b has a larger diameter than that of the first cavity 504a. 
In each of the first piezoelectric resonator 501 and the second piezoelectric resonator 502, by applying electric field between the upper electrode layer 508 and the lower electrode layer 506, the piezoelectric material layer 507 is polarized and distorted, thereby producing mechanical resonance, which is electrically taken out (a function of the resonator). The resonance frequencies of the first piezoelectric resonator 501 and the second piezoelectric resonator 502 are mainly determined, depending on the material, film thickness, and mass loading effect of a vibration section comprised of the frequency adjustment layer 509, the upper electrode layer 508, the piezoelectric material layer 507, the lower electrode layer 506, and the insulating layer 505. If the frequency adjustment layer 509a corresponding to the first piezoelectric resonator and the frequency adjustment layer 509b corresponding to the second piezoelectric resonator have different thicknesses, i.e., one of the frequency adjustment layers 509 is thinner than the other, two piezoelectric resonators having different resonance frequencies can be formed on the same substrate 503.
Note that the different thicknesses of the frequency adjustment layers 509 are provided by using a photolithography technique which typically includes the steps of designing a mask having a portion to be removed and a portion to be left, applying resist, exposing using the designed mask, developing, etching, and removing the resist. See, for example, Japanese Laid-Open Patent Publication No. 2002-359534.
FIG. 11 is a diagram showing an exemplary conventional piezoelectric filter circuit 900 employing a piezoelectric resonator.
In FIG. 11, the conventional the piezoelectric filter circuit 900 comprises series piezoelectric resonators 904a to 904c, parallel piezoelectric resonators 903a to 903d, parallel inductors 905a to 905d, and series inductors 906a and 906b. The series piezoelectric resonators 904a to 904c are connected in series via the series inductors 906a and 906b between an input terminal 902a and an output terminal 902b. The parallel piezoelectric resonators 903a to 903d have first electrodes which are connected to respective connection points between each of the series inductors 906a and 906b and the series piezoelectric resonators 904a to 904c, which are connected in series. The parallel piezoelectric resonators 903a to 903d also have second electrodes which are grounded via the parallel inductors 905a to 905d, respectively.
A portion (a) of FIG. 12 is a diagram showing characteristics of a conventional piezoelectric resonator when used singly. A portion (b) of FIG. 12 is a diagram showing pass characteristics of a conventional piezoelectric filter. The parallel piezoelectric resonators 903a to 903d and the series piezoelectric resonators 904a to 904c theoretically have characteristics which have resonance points 1003 and 1005 where impedance is 0 and antiresonance points 1004 and 1006 where impedance is infinite, respectively. In the portion (a) of FIG. 10, a solid line indicates the characteristics of the parallel piezoelectric resonators 903a to 903d when used singly, and a dashed line indicates the characteristics of the series piezoelectric resonators 904a to 904c when used singly. A difference Δf between the resonance frequency at the resonance point and the antiresonance frequency at the antiresonance point is typically substantially determined based on the piezoelectric material included in the piezoelectric resonator. The piezoelectric filter circuit 900 is configured as follows. The antiresonance point 1004 of the parallel piezoelectric resonators 903a to 903d and the resonance point 1005 of the series piezoelectric resonators 904a to 904c are caused to substantially coincide with each other. The piezoelectric resonators are arranged in a ladder pattern. The parallel inductors 905 and the series inductors 906, which are connected to parasitic inductors and external circuits, are provided.
Thereby, the piezoelectric filter circuit 900 operates as a filter having characteristics which have a lower attenuation pole 1009 (on the lower side of a pass band 1008) at a frequency corresponding to the resonance point 1003 of the parallel piezoelectric resonators 903a to 903d, and a higher attenuation pole 1010 (on the higher side of the pass band 1008) at a frequency corresponding to the antiresonance point 1006 of the series piezoelectric resonators 904a to 904c. See, for example, Japanese Patent No. 2800905.
Note that the first piezoelectric resonator 501 having a high resonance frequency of FIG. 10 corresponds to the series piezoelectric resonators 904a to 904c of FIG. 11, while the second piezoelectric resonator 502 having a low resonance frequency corresponds to the parallel piezoelectric resonators 903a to 903d of FIG. 11.
However, in the above-described conventional filter structure, the cavity 504b of the second piezoelectric resonator 502 having a low impedance has a larger opening area than that of the cavity 504a of the first piezoelectric resonator 501 having a high impedance. Therefore, in the step of etching the substrate 503, reactive gas is more circulated in the cavity 504b having the larger opening area, so that etching proceeds faster in the cavity 504b than in the cavity 504a. Therefore, when etching is performed until the resonance frequency of the first piezoelectric resonator 501 is secured, the cavity 504b of the second piezoelectric resonator 502 penetrates through the substrate 503 to reach the insulating layer 505 (overetching). As a result, the resonance frequency of the second piezoelectric resonator 502 becomes higher than the desired resonance frequency (see FIG. 13).
Therefore, in order to secure the desired resonance frequency of the second piezoelectric resonator 502, the frequency adjustment layer 509b needs to be formed thicker by an amount corresponding to the influence of overetching (see the portion (b) of FIG. 10). However, an increase in the thickness of the frequency adjustment layer 509b leads to a deterioration in a Q value representing the performance of the piezoelectric resonator, and further, a deterioration in the insertion loss of the piezoelectric filter.